Normal aerobic metabolism is associated with the generation of reactive oxygen species (ROS). Certain forms of ROS, such as hydrogen peroxide (H202), function as intracellular second messengers. ROS, however, also cause damage to DNA, proteins and lipids. Aerobic cells cope with the generation of potentially toxic ROS through antioxidant defense mechanisms. For example, catalase, a heine-containing peroxisomal enzyme, catalyzes the conversion of H202 to water and oxygen. H202 is also converted to water by cytosolic and mitochondrial glutathione peroxidase in a reaction which oxidizes glutathione. Despite the importance of these enzymes in controlling ROS, few insights are available regarding the regulation of catalase or glutathione peroxidase activity. Moreover, while failure of these and other antioxidant defenses to control ROS-induced damage is associated with apoptosis, little is known about the mechanisms that determine cell fate in response to oxidative stress. The c-Abl and related Arg nonreceptor tyrosine kinases are activated in the cellular response to oxidative stress. Activation of c-Abl is associated with targeting of c-Abl to mitochondria, release of cytochrome c and induction of apoptosis. Activation of Arg contributes to ROS-induced apoptosis by a mechanism involving tyrosine phosphorylation of the pro-apoptotic Siva-1 protein. The finding that cells deficient in c-Abl or Arg exhibit an attenuated apoptotic response to oxidative stress has supported a mechanism involving both proteins. Our hypothesis is that c-Abl and Arg form heterodimers in the ROS response and that this complex is of importance to the regulation of catalase and glutathione peroxidase. Moreover, our hypothesis is that, if these enzymatic antioxidant mechanisms are unable to control ROS-induced damage, then c-Abl and Arg function as a switch to signal apoptosis. The proposed studies will address these hypotheses and how c-Abl and Arg determine cell fate in the oxidative stress response. The Specific Aims are: 1) To further define the mechanisms responsible for activation of c-Abl and Arg signaling in response to oxidative stress; 2) To determine the role of c-Abl and Arg in the regulation of oxidative stress by catalase; 3) To assess the effects of c-Abl and Arg on activation of glutathione peroxidase; and 4) To define the downstream effectors of c-Abl and Arg in the apoptotic response to oxidative stress.